If you're worried about swivel-eyed loons, try open primaries

People who join political parties exhibit the same altruism as people who work for charities, become special constables or come forward as Olympic volunteers. They are giving up their time and resources to advance a cause they believe in without expecting any reward. Whichever their party, their decision is essentially patriotic, in the sense that they are putting the welfare of the nation, as they see it, before their own convenience.

Some people in every voluntary organisation hold eccentric opinions; but it tells us something about our metropolitan elites that they use the term 'swivel-eyed' almost exclusively to describe mainstream Tory Eurosceptics (the term is a particular favourite of that pitiable old boob Michael White at the Guardian.) Wanting a referendum on leaving the EU is routinely described by bien pensant commentators as 'swivel-eyed' – which makes 82 per cent of the population mad. But hankering after membership of the euro as, say, Peter Mandelson does, is apparently perfectly reasonable.

Not that this is a blog about who said what: an aspect of politics that has always bored me. No, the far larger matter is the vertiginous collapse in the memberships of all parties. Our current model dates from the late nineteenth century. It predates the universal franchise, and casts members in an externally supportive role, like flying buttresses on a cathedral. Activists are supposed to back whatever their MPs do, without expecting much in return.

That model doesn't work any more. The Internet has broken the old monoliths, allowing people to swarm together on an ad hoc, issue-by-issue basis. Voters will gladly join campaigns for 'trade justice', or lower fuel duty, or a referendum on the EU. But they will no longer unconditionally back a political party. The old deference has gone – and not before time.

All party leaders talk about increasing membership. None has succeeded. The only constituencies where the trend is being reversed are those whose local MPs understand that politics has to be open-source. Frinton's Douglas Carswell, for example, who has written a book on what he calls iDemocracy, has doubled the membership of his local Conservative Association – and, significantly, was not affected by the Essex-wide swing to Ukip.

What ought parties to be doing? They should abandon the present model, where members are expected loyally to pay their subscriptions every year, come what may. Instead, they should look at having registered supporters, on the US model, who are able to choose local and national candidates in primaries.

I'll never get tired of saying it: primaries will solve most of our political problems. They will tip power back from the executive to the legislature. They will abolish safe seats, and the complacency that goes with them. They will ensure that we have a genuinely diverse House of Commons. (I don't mean diversity in the BBC sense – people of different colours who all believe more or less the same thing – but genuine pluralism of age, sex, accent, geography, professional background and, above all, opinion.) They will weaken Whips and strengthen voters. They will increase turnout. They will disperse power from remote elites to the general population. They will improve the quality of policy-making. And, for what it's worth, they will ensure that any swivel-eyed people in political parties are even more heavly outnumbered by ordinary citizens than at present.